Soil Carbon

Soil & Global Warming

Surface Water Temperature Biota Climate

Many people believe that the way to tackle global warming is to reduce carbon emissions, and the best way to do that is for the soil to absorb more. After all it has a lot already, and there must be many more ways to 'sequestrate' more carbon.

There is more carbon stored in soil than in the atmosphere (760 billion tonnes) and in vegetation (560 billion tonnes) combined. Total C in terrestrial ecosystems is approximately 3170 gigatons (GT; 1 GT = 1 petagram (Pg) = 1 billion metric tons). Of this amount, nearly 80% (2500 GT) is found in soil. It consists of two distinct components: soil organic carbon (SOC) pool estimated at 1550 Pg and soil inorganic carbon (SIC) pool at 950 Pg. (Lal 2007).

Soil is the planet’s second largest active pool of carbon after the oceans, but its ability to continue to retain the huge amounts of carbon it stores has been weakened in recent decades, largely due to unsustainable land-management practices and changes in land use.

"Globally, a net average decrease of about 60 metric tons of carbon per km2 per year has been reported with a total loss of 3.1 billion (B) tons of carbon from an area of 2 Mkm2 over about 25 years." (Lal 2023)

The big problem

As a result of land use changes, soils are releasing large amounts of carbon into the atmosphere, threatening to undermine reductions in emissions made elsewhere.

"Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts. " (Sanderman, Hengi & Fiske 2017)

Climate extremes have increased since 2005. In 2015 at CoP Paris it was agred to set zero net carbon emissions targets, where emissions have to be equalled by drawdown back into the biosystems. 120btcarbon is drawn down by plants each year, but 130btons is emitted, so we are loosing 10bt /yr. Can we accelerate that draw down? The greenery of the land surface is possibly half what it was 8-10,000ya. UNEP have shown the degradation of forest and grassland and increase of 5b ha of deserts. The potential to draw down more is there, but we need more than the 10bt each year, maybe another 10bt to pull back previous emissions. Can we regenerate that biosystem? The answer is yes, provided these biosystems have water for life - the same search as we spend billions on looking for in outer space. Its not pie in the sky but under our feet! Sustaining our life can only come from the soil. Only water for life in soil can regenerate the trees and grassland.

But even if we pull down 50bt carbon, this will be absorbed by the oceans. It could take 100 years to pull down enough. Snow and coastal rains have gone in areas of America. California is burning now, with the smoke due to reach UK today. We can’t wait 50 years. We have to cool the planet within the next 10 years. Can soil life keep us cool?

Sequestration

Soil carbon sequestration is the process of locking carbon into soils. carbon, Carbon collected by plants and passed to the soil via carbohydrates, also removes carbon dioxide from the atmosphere. Many processes encourage sequestration, particularly composting and mulching.

There are two main channels for carbon capture.

It is not simply a matter of trapping carbon, but of decomposing it - mainly through the process of humification, carried out by worms, (higher oribatid) mites and termites - all of which have arrived in the last 200 million years.
"Traditionally we have assumed that most soil microbes and animals feed on decaying plant tissues. However, in grasslands, where roots make up over 50% of the plant biomass, live roots shed cells, die, and ooze exudates to produce food for soil organisms. Indeed, matter coming into the soil in this way accounts for a significant percentage of carbon inputs" (NERC Soil Biodiversity) .To their list I would add glomalin'

Agriculture

Around a third of the carbon in agriculture, we see above ground crops. Another third is found in the roots of crops, with the other third being the soil biosystem. This is the protozoa, bacteria, fungi, nematodes, springtails, mites and worms. We saw in the introduction that there is 10X weight of healthy animals underneath compared with those above - making a ton and half every cubic metre. Can they deliver for us?

"Normal rates of SOC sequestration on agricultural soils are 300–500 Kg C ha−1 yr−1. High rates are obtained with no-till farming, crop residue retention as mulch, growing cover crops in the rotation cycle and adopting complex farming systems including agroforestry, INM including manuring and through restoration of degraded soils by afforestation". (Lal 2007). Soils under diverse cropping systems generally have a higher SOC pool than those under monoculture (Buyanoski & Wagner 1998). We will pick up on these practices in 'regenerative'

In agriculture remains from the harvest may be incorporated into the soil as part of the cultivation process: ploughing and discing for example. I remember as an agricultural student being considered mad when I complained at burning the stubble. All I could see was a lot of energy going up in smoke. Nowadays, we would see and say that the straw is 'releasing carbon dioxide into the atmosphere rather than sequestrating it'.

Sequestration processes do not lead to a total permanent removal of carbon dioxide. They are rather part of a dynamic equilibrium between the various carbon incorporations and soil processes that release carbon dioxide and methane (and nitrogen oxides) back into the atmosphere. Where the soil is less disturbed by agriculture, carbonaceous rich material does build up in the soil.

The Soilution?

There are many estimates as to how much more carbon soil can store - or sequestrate The estimates of the potential for worldwide carbon sequestration vary from the one quoted by the British Society for Soil Science between 0.8 and 1.5 Gt C per year from the atmosphere to claims that 31Gt could be saved/sequestrated - just 1Gt less than we emit, according to the Footprint Coalition. The claim illuminates a lot of debate going on.

According to the United Nations’ 2022 Emissions Gap Report, there is a 32 gigatonnes gap between current planned emissions reduction globally per year and the amount of carbon that must be cut by 2030 to stay within 1.5°C. Research conducted by Jacqueline McGlade, the former chief scientist at the U.N. environment program shows that soil improvements …would absorb about 31 gigatonnes of carbon dioxide a year. The research was conducted by Downforce Technologies, a commercial organization McGlade now leads.

Their claims have been criticised as they are based on extrapolated estimates of data that is well-known to soil scientists in broad principle, but represents a theoretical maximum that is highly unrealistic. It gives people a false sense of security about ‘plugging the gap’, whereas we should be making sure soil is plugged into all the various warming mitigation strategies.

However, Climate tech startup, Downforce Technologies raises $4.2 million to build soil fertility measurement tools for Africa, which allowed them "to democratise access to the technology and empower a wider audience to make data-driven decisions for soil health and climate action.”

Suddenly soil is up there.. at Davos!! The World Economic Forum said:
"Crop rotation, planting cover crops and using direct drilling could store around 31 gigatonnes of carbon dioxide annually, according to land management company, Downforce Technologies. And that would make a sizable impact since it is close to the 32 gigatonnes gap that exists between planned emissions reduction globally per year, and the amount of carbon that must be cut by 2030 to stay within 1.5C global heating" Podcast

UK Initiatives

"Tree planting in the UK is estimated to need to increase to thirty thousand hectares per year between now and 2050 (which is equivalent to 90 - 120 million trees planted per year). Increasing the efficiency of existing forestry management to increase current yields will also be required, through improved silvicultural practices. As part of this tree planting will also need to occur on existing crop and grassland, and the presence of hedgerows will need to increase to 181,000 hectares by 2050. This has the potential to deliver around 6 MtCO2e (mega tonnes of CO2 equivalent). However, in addition to this goal, the principles of natural capital management indicate that tree planting approaches need to consider more than just carbon sequestration, as factors such as ecosystem function and resilience are likely to be as important in terms of overall sustainability and ecosystem service delivery. More Forests and temperature

Improved peatland management is also a key component of future climate friendly land management. Peat is a carbon store of significant importance. The restoration of peatland systems, degraded through modification in order to support food and fibre production, will be essential to return these systems to carbon sinks rather than carbon sources. This will involve blocking drains and rewetting bogs, which may increase methane emissions in the short-term. Nevertheless, this is an essential action to return this habitat to functional status. Moving production away from such sensitive habitats, allowing them to act as carbon sinks, once returned to functionality, can allow production to happen elsewhere, offsetting the overall impact of necessary food production.

Cultivation of biomass crops, for the purposes of bioenergy production, may present an opportunity to integrate climate-smart actions into existing production scenarios. Producing fuel in this manner could help to reduce the reliance on fossil fuels, whilst increasing the potential for plant-based carbon capture and storage. Furthermore, this could be seen as an approach to draw CO2 from the atmosphere, which can then be captured using man-made carbon capture technologies during the combustion process. In this manner, this approach differs from other nature-based approaches, as in this system carbon could be effectively removed from the C-cycle, which is not necessarily the case in the long-term with other interventions."

Much of these land approaches are being encouraged by ELMS. The weakness is that it presumes we are going to import more cheap food, using other people's land, labour and water. ie the carbon footprint has trodden elsewhere.

The drive to store more carbon in soil may be simplistic, reductionist and limited, but it is the first time in the last 50 years where people are looking to seriously invest in soil. We should make the most of that as it brings about new ways of looking at the soil, and with that - hopefully - healthier soil. We should point out the added benefits of soil organic matter to cooling reactions on the surface, water and temperature directly, showing these added bonuses to tackle 'neglected 1/3rd' of global warming.

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